This invention relates to a method for the production of reinforced composites from fibrous reinforcing materials and compositions which are photopolymerisable and thermosettable, and to the composites obtained by this method.
Composite structures are commonly made by impregnating fibrous materials, such as fibers of paper, glass, an aromatic polyamide, and carbon, metal filaments, or whiskers, usually in the form of layers, with a solution of a solid thermosettable resin in a volatile solvent and with a heat-activated curing agent for the resin, causing the resin to solidify by evaporation of the solvent, and, when desired, curing the resin composition in the resultant so-called "prepreg" by the action of heat. Frequently, the prepregs are stacked before heat-curing, so that a multilayer laminate is formed.
Composite structures may also be prepared from films of a thermosettable resin composition by the method described in British Pat. No. 1 299 177, which comprises laying a film of the resin composition on a fibrous reinforcement and applying heat and pressure so that the resin composition flows about the fibers but remains curable, and then heating further when desired so that the resin is cured by the heat-activated curing agent. This procedure is particularly convenient when unidirectional fibrous reinforcement is to be used, especially if the fibres are short and/or light, because there is less tendency for the fibres to become displaced and the reinforcing effect thereby become irregularly distributed.
Both these methods, however, suffer from certain drawbacks. If a solvent is used to dissolve components of the thermosettable resin composition, it is not always possible to remove all traces of the solvent before heat-curing takes place, and in consequence the cured composite may contain voids caused by evaporation of residual solvent. Solvents may cause difficulties due to their toxicity or flammability or to pollution. If a film adhesive is used, it is usually cast from a liquid thermosettable resin composition and this is then advanced to the solid state; such a process adds considerably to the cost of the composite. Both methods also require a considerable expenditure of heat energy, either to evaporate the solvents or to advance the resin.
We have now found that fibrous reinforced composites may be made by a procedure which substantially avoids the inconveniences just mentioned. In this novel method, a liquid composition containing an epoxide resin is photopolymerized to form an essentially solid continuous film by exposure to actinic radiation, optionally in the presence of a catalyst for the photopolymerisation, but without thermally crosslinking it; the film so obtained is then contacted with fibrous reinforcing material, usually with the application of heat and/or pressure, such that a coherent structure is formed. The period of heating can be very short, as there need be no solvent to evaporate and the film need not be thick. It is not necessary to convert immediately the photopolymerised composition distributed on the fibrous reinforcing material into the fully cured, insoluble, and infusible C-stage; often, it can be changed into the still fusible B-stage, or remain in the A-stage, and, when desired, e.g., after the prepreg has been formed into some desired configuration, fully cured by heating to form the reinforced composite.